Heat pipe networks for heat removal, such as heat removal from nuclear reactors, and associated systems and methods

ABSTRACT

Nuclear reactor systems and associated devices and methods are described herein. A representative nuclear reactor system includes a heat pipe network having an evaporator region, an adiabatic region, and a condenser region. The heat pipe network can define a plurality of flow paths having an increasing cross-sectional flow area in a direction from the evaporator region toward the condenser region. The system can further include nuclear fuel thermally coupled to at least a portion of the evaporator region. The heat pipe network is positioned to transfer heat received from the fuel at the evaporator region, to the condenser region. The system can further include one or more heat exchangers thermally coupled to the evaporator region for transporting the heat out of the system for use in one or more processes, such as generating electricity.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/915,467, filed Oct. 15, 2019, and titled “HEAT PIPE NETWORKS FORHEAT REMOVAL, SUCH AS HEAT REMOVAL FROM NUCLEAR REACTORS, AND ASSOCIATEDSYSTEMS AND METHODS,” which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present technology relates generally to nuclear reactors andassociated systems and methods and, more particularly, to nuclearreactors having heat pipes for removing heat generated by a nuclearfuel.

BACKGROUND

Power plants come in many different shapes and sizes. Large power plantscan be used to provide electricity to a geographic area, whereasrelatively small power plants can be used to power, for example, localareas, submarines, space craft, etc. In addition to providingelectricity, power plants can be used for a myriad of additional ordifferent purposes, from desalinating seawater to creating nuclearisotopes for medical purposes. Similarly, the types of power plants thatare available cover a wide spectrum of technologies includinggas-powered, coal-fired, and nuclear-powered, to name a few.

To date, nuclear reactor designs that use heat pipes as a means for heatremoval from a core of nuclear material maintain constant heat pipe flowarea in discreet unconnected heat pipes. For a given reactor coregeometry, heat removal is limited by the total effective heat pipe flowarea and the fraction of the core volume occupied by the heat pipes. Forexample, a larger number of heat pipes will increase the total effectiveflow area and heat removal capacity but will also displace fuel andother core material, which will reduce the heat production potential ofthe core.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present technology can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale. Instead, emphasis is placed on clearlyillustrating the principles of the present technology.

FIG. 1 is a partially schematic side cross-sectional view of a nuclearreactor system configured in accordance with embodiments of the presenttechnology.

FIGS. 2A and 2B are a top cross-sectional view and an enlarged topcross-sectional view, respectively, of a reflector region and a coreregion of the nuclear reactor system configured in accordance withembodiments of the present technology.

FIG. 2C is a cross-sectional isometric view of a heat pipe of thenuclear reactor system configured in accordance with embodiments of thepresent technology.

FIG. 3 is a flow diagram of a process or method for forming a nuclearfuel on a heat pipe of the nuclear reactor system using a hot isostaticpressing (HIP) process in accordance with embodiments of the presenttechnology.

FIGS. 4A and 4B are enlarged top cross-sectional views of the portion ofthe nuclear reactor system shown in FIG. 2B and illustrating a heatexchanger of the nuclear reactor system configured in accordance withembodiments of the present technology.

FIGS. 5A-5C are a top cross-sectional view, an enlarged topcross-sectional view, and a further enlarged top cross-sectional view,respectively, of the core region of a nuclear reactor system configuredin accordance with additional embodiments of the present technology.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed generally toward nuclearreactor systems. In several of the embodiments described below, arepresentative nuclear reactor system includes a network ofinterconnected heat pipes having an evaporator region, an adiabaticregion, and a condenser region. The heat pipes are configured to containa working fluid, and the network of heat pipes can define an increasingcross-sectional flow area for the working fluid in a direction from theevaporator region toward the condenser region. The system can furtherinclude a nuclear fuel, such as a fissile material, configured togenerate heat and thermally coupled to at least a portion of theevaporator region. The network of heat pipes is configured to transferthe heat from the evaporator region to the condenser region. The systemcan further include one or more heat exchangers thermally coupled to thecondenser region for transporting the heat out of the system for use inone or more processes, including, but not limited to, generatingelectricity.

In one aspect of the present technology, the increasing flow area of theheat pipes can increase the heat removal capacity of the heat pipes ascompared to, for example, conventional heat pipes having a single pipeof constant flow area. In some embodiments, the heat pipes can branch orbifurcate in the direction from the evaporator region toward thecondenser region to increase the flow area of the network of heat pipes.

In some embodiments, the network of heat pipes can be provided in a heatpipe layer, and the nuclear reactor system can include a plurality ofstacked heat pipe layers. In one aspect of the present technology, thenumber of heat pipe layers can be varied in accordance with, forexample, the amount of fuel in the system, and thus the power/heatoutput of the system. In another aspect of the present technology, theheat pipe layers can be loosely coupled to a common frame and/or otherones of the heat pipe layers. This can allow the heat pipe layers toexpand/contract independently due to heat and/or irradiation—therebyreducing mechanical stress on the heat pipe layers, the frame, and/orother components of the system. In some embodiments, to refuel thesystem, one or more of the heat pipe layers—and the fuel attached to orotherwise associated therewith—can be removed and replaced and/orshifted to another location within the stack of heat pipe layers, forexample, without requiring the entire network of heat pipes to beremoved.

In some embodiments, the fuel can be directly attached to the evaporatorregion of the heat pipes, which can provide a high thermal couplingbetween the fuel and the heat pipes. For example, the fuel can be formedon the heat pipes using a hot isostatic pressing (HIP) process.

Certain details are set forth in the following description and in FIGS.1-5C to provide a thorough understanding of various embodiments of thepresent technology. In other instances, well-known structures,materials, operations, and/or systems often associated with nuclearreactors, heat pipes, heat exchangers, etc., are not shown or describedin detail in the following disclosure to avoid unnecessarily obscuringthe description of the various embodiments of the technology. Those ofordinary skill in the art will recognize, however, that the presenttechnology can be practiced without one or more of the details set forthherein, and/or with other structures, methods, components, and so forth.

The terminology used below is to be interpreted in its broadestreasonable manner, even though it is being used in conjunction with adetailed description of certain examples of embodiments of thetechnology. Indeed, certain terms may even be emphasized below; however,any terminology intended to be interpreted in any restricted manner willbe overtly and specifically defined as such in this Detailed Descriptionsection.

The accompanying Figures depict embodiments of the present technologyand are not intended to be limiting of its scope unless expresslyindicated. The sizes of various depicted elements are not necessarilydrawn to scale, and these various elements may be enlarged to improvelegibility. Component details may be abstracted in the Figures toexclude details such as position of components and certain preciseconnections between such components when such details are unnecessaryfor a complete understanding of how to make and use the presenttechnology. Many of the details, dimensions, angles and other featuresshown in the Figures are merely illustrative of particular embodimentsof the disclosure. Accordingly, other embodiments can have otherdetails, dimensions, angles and features without departing from thepresent technology. In addition, those of ordinary skill in the art willappreciate that further embodiments of the present technology can bepracticed without several of the details described below.

FIG. 1 is a partially schematic side cross-sectional view of a nuclearreactor system 100 (“system 100”) configured in accordance withrepresentative embodiments of the present technology. In the illustratedembodiment, the system 100 includes a reactor container 102 and aradiation shield container 104 surrounding/enclosing the reactorcontainer 102. In some embodiments, the reactor container 102 and theradiation shield container 104 can be roughly cylinder-shaped orcapsule-shaped. The system 100 further includes a plurality of heat pipelayers 106 within the reactor container 102. In the illustratedembodiment, the heat pipe layers 106 are spaced apart from and stackedover one another. In some embodiments, the heat pipe layers 106 can bemounted/secured to a common frame 109, a portion of the reactorcontainer 102 (e.g., a wall thereof), and/or other suitable structureswithin the reactor container 102. In other embodiments, the heat pipelayers 106 can be directly stacked on top of one another such that eachof the heat pipe layers 106 supports and/or is supported by one or moreof the other ones of the heat pipe layers 106.

In the illustrated embodiment, the system 100 further includes a shieldor reflector region 114 at least partially surrounding a core region116. The heat pipe layers 106 can be circular, rectilinear, polygonal,and/or can have other shapes, such that the core region 116 has acorresponding three-dimensional shape (e.g., cylindrical, spherical,etc.). In some embodiments, the core region 116 is separated from thereflector region 114 by a core barrier 115, such as a metal wall. Thecore region 116 can include one or more fuel sources, such as fissilematerial, for heating the heat pipe layers 106. The reflector region 114can include one or more materials configured to contain/reflect productsgenerated by burning the fuel in the core region 116 during operation ofthe system 100. For example, the reflector region 114 can include aliquid or solid material configured to reflect neutrons and/or otherfission products radially inward toward the core region 116. In someembodiments, the reflector region 114 can entirely surround the coreregion 116. In other embodiments, the reflector region 114 may onlypartially surround the core region 116. In some embodiments, the coreregion 116 can include a control material 117, such as a moderatorand/or coolant. The control material 117 can at least partially surroundthe heat pipe layers 106 in the core region 116 and can transfer heattherebetween. In some embodiments, as described in greater detail below,the control material 117 can be a liquid moderator (e.g., a liquid metalalloy, a liquid metal hydride) configured to control the reactivity ofthe system 100.

In the illustrated embodiment, the system 100 further includes at leastone heat exchanger 108 positioned around the heat pipe layers 106. Theheat pipe layers 106 can extend from the core region 116 and at leastpartially into the reflector region 114, and are thermally coupled tothe heat exchanger 108. As described in greater detail below withreference to FIGS. 4A and 4B, the heat exchanger 108 can be positionedoutside of or partially within the reflector region 114. As described ingreater detail below with reference to FIGS. 2A-5C, the heat pipe layers106 can each include an array of heat pipes that provide a heat transferpath from the core region 116 to the heat exchanger 108. Duringoperation of the system 100, the fuel in the core region 116 can heatand vaporize a fluid within the heat pipes in the heat pipe layers 106,and the fluid can carry the heat to the heat exchanger 108.

In some embodiments, the heat exchanger 108 can include one or morehelically-coiled tubes that wrap around the heat pipe layers 106. Thetubes of the heat exchanger 108 can include or carry a working fluid(e.g., a coolant such as water or another fluid) that carries the heatfrom the heat pipe layers 106 out of the reactor container 102 and theradiation shield container 104 for use in generating electricity, steam,etc. For example, in the illustrated embodiment the heat exchanger 108is operably coupled to a turbine 110, a generator 111, a condenser 112,and a pump 113. As the working fluid within the heat exchanger 108increases in temperature, the working fluid may begin to boil andvaporize. The vaporized working fluid (e.g., steam) may be used to drivethe turbine 110 to convert the thermal potential energy of the workingfluid into electrical energy via the generator 111. The condenser 112can condense the working fluid after it passes through the turbine 110,and the pump 113 can direct the working fluid back to the heat exchanger108, where it can begin another thermal cycle.

FIGS. 2A and 2B are a top cross-sectional view and an enlarged topcross-sectional view, respectively, of the reflector region 114 and thecore region 116 configured in accordance with representative embodimentsof the present technology. More specifically, FIG. 2A illustrates one ofthe heat pipe layers 106. In the illustrated embodiment, the heat pipelayer 106 includes a network of at least partially interconnected heatpipes 220. The heat pipes 220 can be separate tubes, channels formedin/between one or more solid members, and/or other structures configured(e.g., positioned and shaped) to provide one or more flow paths (e.g.,for a working fluid contained therein).

The heat pipes 220 can have a branching/tapering arrangement in whichthe heat pipes 220 bifurcate as they extend radially outward in adirection from the reactor container 102 toward the reflector region 114(e.g., in a direction away from a central axis C of the heat pipe layer106 as indicated by arrow R in FIG. 2A). More specifically, referring toFIG. 2B, the heat pipes 220 can comprise first portions 221 that arepositioned radially inward of and bifurcate into (e.g., divide into,connect to, etc.) second portions 222. Likewise, the second portions 222can be positioned radially inward of and bifurcate into third portions223, the third portions 223 can be positioned radially inward of andbifurcate into fourth portions 224, and so on. In some embodiments, theheat pipes 220 include more or fewer bifurcations. The heat pipes 220can all be fluidly connected together, or two or more different subsetsof the heat pipes 220 can be fluidly connected together. For example, asubset or branch of the heat pipes 220 (e.g., a wedge-shaped branchshown in FIG. 2B) including one of the first portions 221 and itsprogeny—for example, two of the second portions 222, four of the thirdportions 223, eight of the fourth portions 224, and so on—can beconnected together. Adjacent wedge-shaped branches can be independent ofeach other, or individual wedge-shaped branches can be connectedtogether (e.g., at the entrances of the adjacent first portions 221).

FIG. 2C is a cross-sectional isometric view of one of the heat pipes 220configured in accordance with embodiments of the present technology. Inthe illustrated embodiment, the heat pipe 220 includes an outer wall 230having an outer surface 232 a and an inner surface 232 b, and defining achannel 234. The heat pipe 220 includes a working fluid (not shown) thatis contained within the channel 234. The working fluid can be atwo-phase metal (e.g., liquid and vapor phase) such as, for example,sodium or potassium. The wall 230 can be formed from any suitablystrong, thermally conductive, and neutronic resistant material such as,for example, one or more metal or ceramic materials. In a particularembodiment, the wall 230 comprises molybdenum alloy. In the illustratedembodiment, the wall 230 has a generally square cross-sectional shapewhile, in other embodiments, the wall 230 can have a circular,rectangular, polygonal, irregular, or other cross-sectional shape.

The heat pipe 220 further includes a first mesh or wick 236 extendingalong/over a portion of the inner surface 232 b, such as a lower/floorportion of the inner surface 232 b (e.g., relative to gravity). The heatpipe 220 can further include a second mesh or wick 238 extendingalong/over all or a portion of the rest of the inner surface 232 b andthe first wick 236. In some embodiments, the first wick 236 is a coarsewick capable of relatively high throughput of the working fluid comparedto the second wick 238. The second wick 238 can be a fine wickconfigured to pump the working fluid against a larger pressure gradientthan the first wick 236, but for shorter distances that the first wick236. Accordingly, the first and second wicks 236, 238 can together forma compound wick in which (i) the first wick 236 allows for long distanceflow of the working fluid and (ii) the second wick 238 allows forlocalized flow of the working fluid. In other embodiments, the heat pipe220 can include other wick arrangements (e.g., compound or single wickarrangements) for promoting the flow of the working fluid through thechannel 234 of the heat pipe 220.

Referring to FIGS. 2A-2C together, the heat pipes 220 extend radiallyoutward from the central axis C at least partially through (i) the coreregion 116 and the control material 117 therein and (ii) the reflectorregion 114. More specifically, in some embodiments the first, second,and third portions 221-223 of the heat pipes 220 are positioned withinthe core region 116, while the fourth portions 224 extend through thereflector region 114. In the illustrated embodiment, the fourth portions224 of the heat pipes 220 extend entirely through the reflector region114 such that an outward terminus 225 of the heat pipes 220 ispositioned radially outside of the reflector region 114.

Fuel 226 (FIG. 2B) is positioned around/proximate to the heat pipes 220in the core region 116 (e.g., on the first, second, and third portions221-223 of the heat pipes 220). In some embodiments, the fuel 226 can bea solid metallic fuel including fissile material, such as a uraniummolybdenum alloy having high thermal conductivity and a high density offissile material. In some embodiments, the fuel 226 can be inserted intoslots (not shown) along the heat pipes 220 or the structure supportingthe heat pipes 220 (e.g., the frame 109 shown in FIG. 1 ) such thatthere is no gap or only a small gap between the walls 330 of the heatpipes 220 and the fuel 226. In such embodiments, the slots can cover thefuel 226 and act as a cladding that inhibits or even prevents growth ofthe fuel 226 and/or the escape of fission products. In otherembodiments, the fuel 226 can be directly formed on/attached to the heatpipes 220 using, for example, a hot isostatic pressing (HIP) process,sintering, additive manufacturing, and/or other suitable process.

More specifically, FIG. 3 is a flow diagram of a process or method 360for forming the fuel 226 on the heat pipes 220 (e.g., forming a nuclearreactor component) using a HIP process in accordance with embodiments ofthe present technology. At block 362, the method 360 includespositioning one or more of the heat pipes 220 in a pressure vessel. Atblock 364, the method 360 includes providing/depositing a powdered fuelmaterial (e.g., a fissile material) on a portion of the heat pipes 220.For example, the fuel 226 can be provided as a metal powder (e.g., apowder of uranium molybdenum alloy) on the walls 230 of the heat pipes220 to be located in the core region 116. The fuel 226 can be providedon the heat pipes 220 before or after the heat pipes 220 are positionedin the pressure vessel. At block 366, the method 360 includes increasinga pressure and a temperature in the pressure vessel to solidify/compressthe fuel material and directly attach the fuel material to the heatpipes 220. For example, the increased temperature and pressure cancompress the powdered fuel 226 into a compact solid on the walls 330 ofthe heat pipes 220. In some embodiments, increasing the pressure in thepressure vessel includes increasing an isostatic gas pressure in thepressure vessel by pumping an inert gas (e.g., argon) into the pressurevessel that does not react with the material of the heat pipes 220 orthe fuel 226. In some embodiments, the temperature can be greater thanabout 450° C., greater than about 1000° C., greater than about 1300° C.,or hotter. In some embodiments, the pressure can be greater than about20 MPa, greater than about 50 MPa, greater than about 100 MPa, orhigher.

Accordingly, referring to FIGS. 1-3 together, in one aspect of thepresent technology the fuel 226 can be physically attached to/integratedwith the walls 330 of the heat pipes 220, which can provide a highthermal coupling between the fuel 226 and the heat pipes 220. In someembodiments, a cladding material can be formed around the fuel 226 usingthe same or a different process for attaching the fuel 226 to the heatpipes 220. For example, a HIP process can also be used to form acladding layer around the fuel 226.

During system operation (e.g., after the fuel 226 and heat pipes 220have been installed in the system 100), the fuel 226 generates heat thatis transferred to the heat pipes 220 to evaporate/vaporize the workingfluid therein. The first, second, and third portions 221-223 of the heatpipes 220 in the core region 116 can define an evaporator region of theheat pipes 220 in which the working fluid is heated andevaporated/vaporized. The evaporated working fluid then flows radiallyoutward through the channels 234 along the fourth portions 224 of theheat pipes 220 (e.g., an adiabatic region of the heat pipes 220) throughthe reflector region 114 toward the outward termini 225 of the heatpipes 220 (e.g., a condenser region of the heat pipes 220) where theworking fluid cools and condenses. The first and second wicks 236, 238are configured to transport the condensed/cooled working fluid againstthe pressure gradient in the heat pipes 220 to the evaporator region ofthe heat pipes 220 where the working fluid can be heated and vaporizedonce again. Accordingly, in some embodiments heat is deposited into theevaporator region of the heat pipes 220, removed from the condenserregion of the heat pipes 220, and neither removed from nor added to theheat pipes 220 in the adiabatic region. The vapor mass flow rate of theworking fluid can increase over the length of the evaporator region anddecrease over the length of the condenser region.

In one aspect of the present technology, the branching (e.g.,fractionalizing, bifurcating) arrangement of the heat pipes 220 providesan increasing flow area (e.g., cross-sectional flow area) in the radialdirection (e.g., in the direction indicated by the arrow R in FIG. 2A).That is, the heat pipes 220 originate from near a common point (e.g.,the central axis C) and grow in flow area by branching from the firstportions 221 to the second portions 222, from the second portions 222 tothe third portions 223, and so on. This arrangement can increase theheat removal capacity of the heat pipes 220 as compared to conventionalheat pipes having a single pipe of constant flow area.

More specifically, there are two at least two properties that affect theheat removal capacity of a heat pipe—the capillary limit and the soniclimit. The capillary limit occurs when the forces from the pressuredifference across the length of the heat pipe exceed the capillaryforces within the wick of the heat pipe, preventing the coolant liquidfrom reaching the end of the heat pipe. This limit can be addressed byimproving the wick to increase the capillary force, or by improvementsto the vapor flow path to reduce the pressure difference. The soniclimit represents the velocity when choked flow will occur, at whichpoint more fluid cannot be transported by the heat pipe. This limit canbe addressed by larger flow areas or lower power. The arrangement of theheat pipes 220 increases the vapor and liquid flow area along the lengthof the heat pipes 220 compared to conventional heat pipes having aconstant flow area, without exceeding the capillary and sonic limits,and therefore provides improved heat transfer. Specifically, theincrease in flow area limits the increase in vapor velocity thattypically occurs in the evaporator section of a heat pipe. This allowsthe branching heat pipes 220 to draw more heat before encountering anoperational limit.

In another aspect of the present technology, the branching heat pipes220 also have an increasing surface area in the radial direction (e.g.,in the direction indicated by the arrow R in FIG. 2A). This allows for arelatively high effective contact surface area between the heat pipes220 and the fuel 226—providing more surface area to attach the fuel 226to—and thereby allowing for the fuel 226 to be made thinner and to bespread out across the larger surface area. Further, the branching heatpipes 220 can provide additional surface area, which in turn providesmore radially uniform cooling in the condenser region near the outwardtermini 225.

Branching also reduces the number of separate heat pipes 220 needed inthe core region 116. For example, a single one of the branching heatpipes 220 can cover a section of the core region 116 with multiple flowexits from the evaporator region. Accordingly, the branching heat pipes220 reduce the volume of pipes within the core region 116 for the sameheat removal rate (e.g. the same evaporator exit flow area). Becausefewer pipes can be used, the heat exchanger 108 (FIG. 1 ) can be mademore compact and/or more of the fuel 226 and/or the control material 117(e.g., a moderator) can be included in the core region 116. In sum, thepresent technology provides a means for increasing the heat removalcapacity of the system 100 while reducing a fraction of the volume ofthe core region 116 needed for heat removal.

FIGS. 4A and 4B are enlarged top cross-sectional views of the portion ofthe system 100 shown in FIG. 2B and illustrating the heat exchanger 108in accordance with embodiments of the present technology. Referringfirst to FIG. 4A, in some embodiments the heat exchanger 108 can bepositioned within the reflector region 114 proximate to the outwardtermini 225 of the heat pipes 220 (e.g., proximate to the condenserregion of the heat pipes 220). In some such embodiments, the reflectorregion 114 comprises a liquid 440, such as a liquid metal material(e.g., a liquid metal alloy, a liquid metal hydride) configured toreflect neutrons inward toward the core region 116. In some embodiments,the heat exchanger 108 can be directly coupled/attached to the heatpipes 220 and/or their supporting structure while, in other embodiments,the heat exchanger 108 can be spaced apart from the heat pipes 220 inthe reflector region 114. For example, the liquid 440 can act as a heattransfer medium for transferring heat from the heat pipes 220 to theheat exchanger 108. In some embodiments, the core barrier 115 canseparate the liquid 440 in the reflector region 114 from the controlmaterial 117 in the core region 116, and the heat pipes 220 canpenetrate through the core barrier 115. In one aspect of the presenttechnology, the thermal properties of the liquid 440 in the reflectorregion 114 can help distribute heat around the heat exchanger 108, andthereby inhibit failure of the heat exchanger 108 from a failure of anyof the heat pipes 220.

Referring next to FIG. 4B, in some embodiments the heat exchanger 108can be positioned radially outside of the reflector region 114 proximateto the outward termini 225 of the heat pipes 220 (e.g., proximate to thecondenser region of the heat pipes 220). In some such embodiments, thereflector region 114 comprises a solid material 442 configured toreflect neutrons inward toward the core region 116. Accordingly, theheat pipes 220 can penetrate through the solid material 442.

Referring to FIGS. 1-2C, 4A, and 4B together, in some embodiments thecore region 116 can include the control material 117 at least partiallysurrounding the heat pipes 220 and the fuel 226 in the core region 116.In the illustrated embodiment, the control material 117 terminates atthe edge of the core region 116 and thus entirely surrounds the fuel226. In other embodiments, the control material 117 can extend at leastpartially into the reflector region 114 and can act as part of thereflector. In yet other embodiments, the control material 117 can beomitted.

The control material 117 can be a moderator configured to control thereactivity of the system 100 and/or a coolant configured to distributeheat between the heat pipe layers 106. In some embodiments, the controlmaterial 117 can be a liquid moderator including, for example, a metalhydride that can be used to control the reactivity of the fuel 226 byadding or removing hydrogen from the system 100. More particularly, thecontrol material 117 can be a mixture of calcium hydride (e.g., CaH₂),calcium bismuth (Bi—Ca), and/or other suitable compounds. Such mixturescan undergo reversible conversions between metal and metal hydride basedon the hydrogen content of the system 100 and, accordingly, can be usedas a moderator to control a reactivity of the fuel 226 based on thestate of the control material 117. For example, in some embodiments thesystem 100 can include a hydrogen control system 119 (FIG. 1 )configured to vary a hydrogen content level of the core region 116 tothereby control the state of the control material 117 and thus itsmoderating properties. The control material 117 can also act as acoolant to evenly distribute heat between the heat pipe layers 106. Forexample, if one or more of the heat pipes 220 fail, the control material117 can help transfer heat from the fuel 226 around the failed one ofthe heat pipes 220 to other, functioning heat pipes 220 for removal fromthe system 100. In one aspect of the present technology, the controlmaterial 117 therefore reduces thermal-gradient induced stresses arisingfrom heat pipe failures and simplifies the structural design of the coreregion 116.

In addition to or alternatively to the control material 117, the system100 can include one or more control rods (not shown), such as a boroncontrol rod, configured to absorb (e.g., poison) neutrons in the coreregion 116 that may otherwise induce fission of the fuel 226. Suchcontrol rods can help facilitate startup and shutdown of the system 100.

FIGS. 5A-5C are a top cross-sectional view, an enlarged topcross-sectional view, and a further enlarged top cross-sectional view,respectively, of a portion of one of the heat pipe layers 106 positionedat the core region 116 in accordance with additional embodiments of thepresent technology. Some of the features and associated functionality ofthe heat pipe layer illustrated in FIGS. 5A and 5B can be generallysimilar to or identical to the features and/or functionality of the heatpipe layer described in detail with respect to FIGS. 2A-4B. For example,referring to FIGS. 5A-5C together, the heat pipe layer 106 includes anetwork of interconnected heat pipes. In the illustrated embodiment,however, the heat pipe layer 106 includes (i) arterial heat pipes 520(e.g., arterial flow paths; identified individually as first throughthird arterial heat pipes 520 a-520 c) extending radially outward in adirection indicated by the arrow R away from the central axis C of theheat pipe layer 106 (FIG. 5A) and (ii) off-branching heat pipes 552(e.g., off-branching flow paths) extending circumferentially relative tothe central axis C and connecting two or more of the arterial heat pipes520.

In the illustrated embodiment, the first arterial heat pipes 520 aextend farther radially inward than the second and third arterial heatpipes 520 b, c, and the second arterial heat pipes 520 b extend fartherradially inward than the third arterial heat pipes 520 c. Moreover, thearterial heat pipes 520 can be generally equally spacedcircumferentially about the central axis C, and the number of the thirdarterial heat pipes 520 c can be greater (e.g., double) the number ofthe second arterial heat pipes 520 b, the number of second arterial heatpipes 520 b can be greater (e.g., double) the number of the firstarterial heat pipes 520 a, and so on. Accordingly, the arterial heatpipes 520 can form an alternating/interleaved arrangement in which eachof the first arterial heat pipes 520 a is directly adjacent to two(e.g., a pair) of the second arterial heat pipes 520 b, each of thesecond arterial heat pipes 520 b is directly adjacent two of the thirdarterial heat pipes 520 c, and so on. In other embodiments, the coreregion 116 can have more or fewer of the arterial heat pipes 520, and/orthe arterial heat pipes 520 can be arranged differently (e.g.,asymmetrically). The arterial heat pipes 520 contain a working fluid andprovide a heat removal path at least partially through the reflectorregion 114 (FIG. 1 ) to the heat exchanger 108 (FIG. 1 ).

The off-branching heat pipes 552 each extend between and connect anadjacent pair of the arterial heat pipes 520 (e.g., a circumferentiallyadjacent pair). The arterial heat pipes 520 and the off-branching heatpipes 552 (collectively “heat pipes 520, 552”) can all be fluidlyconnected together, or two or more different subsets of the heat pipes520, 552 can be fluidly connected together. For example, a subset orbranch of the heat pipes 520, 552 can be connected together, such as awedge-shaped branch shown in FIG. 5B including one of the first arterialheat pipes 520 a, an adjacent two of the second arterial heat pipes 520b, an adjacent four of the third arterial heat pipes 520 c, and so on.

In the illustrated embodiment, the off-branching heat pipes 552 eachinclude one or more (e.g., two or more) evaporator heads 554. Referringto FIG. 5C, the fuel 226 can be positioned around/proximate to theevaporator heads 554. For example, in the illustrated embodiment thefuel 226 is directly attached to the evaporator heads 554 (e.g., via aHIP process) such that there is no gap or a small gap between theevaporator heads 554 and the fuel 226. In other embodiments, the fuel226 can be inserted into slots on the evaporator heads 554. In someembodiments, the evaporator heads 554 can be evenly spaced apart fromanother along the off-branching heat pipes 552 such that the number ofevaporator heads 554 increases in the radial direction as the lengthand/or number of the off-branching heat pipes 552 increases.

Referring again to FIGS. 5A-5C together, during operation of the system100, the fuel 226 generates heat that is transferred to the evaporatorheads 554 to evaporate/vaporize the working fluid therein and/or in theoff-branching heat pipes 552. The evaporated working fluid then flowscircumferentially toward one or both of the connected arterial heatpipes 520, and then radially outward along the arterial heat pipes 520through the reflector region 114 (FIG. 1 ) toward the heat exchanger 108(FIG. 1 ) where the working fluid cools, condenses, and transfers heatto the heat exchanger 108. The heat pipes 520, 552 can include wicks(e.g., as described in detail with reference to FIG. 2C) configured totransport the condensed/cooled working fluid against the pressuregradient in the heat pipes 520, 552 to the evaporator heads 554 wherethe working fluid can be heated and vaporized once again. Accordingly,the heat pipes 520, 552 remove heat from the fuel 226 and transport theheat radially outward toward the heat exchanger 108 (FIG. 1 ).

Similar to the embodiments described above with reference to FIGS.2A-4B, the arrangement of the heat pipes 520, 552 provides an increasingflow area in the radial direction (e.g., in the direction indicated bythe arrow R). That is, the network of heat pipes 520, 552 originatesfrom near a common point (e.g., the central axis C) and grows in flowarea as the number and/or length of heat pipes 520, 552 increases in theradial direction. As described in detail above, this arrangement canincrease the heat removal capacity of the heat pipes 520, 552 ascompared to conventional heat pipes having a single pipe of constantflow area. Likewise, in some embodiments the control material 117 can atleast partially surround the heat pipes 520, 552 and the fuel 226 in thecore region 116.

Referring again to FIG. 1 , in other embodiments the heat pipe layers106 can each comprise one or more disk-shaped (e.g., circular) planarheat pipes. In some embodiments, fuel elements having a correspondingflat disk shape can be attached to the heat pipes in the core region116. That is, the heat pipe layers 106 can be positioned betweencorresponding flat fuel elements. Such embodiments can provide arelatively simple arrangement that still provides a relatively highvapor flow area and high surface area for contact with the fuel.

Referring to FIGS. 1-5C together, the heat pipe layers 106 can each beidentical. In other embodiments, the configurations of the heat pipelayers 106 can be varied. For example, some of the heat pipe layers 106can include the network of heat pipes 220 shown in FIGS. 2A-2C, whilesome of the heat pipe layers 106 include the network of heat pipes 520,552 shown in FIGS. 5A-5C, and/or while some of the heat pipe layers 106include flat disk-shaped heat pipes.

In one aspect of the present technology, the number of heat pipe layers106 can be varied to vary the amount of fuel 226 in the system 100, andthus the power/heat output of the system 100. In another aspect of thepresent technology, the heat pipe layers 106 can be loosely coupled(e.g., via a plurality of flexible joints) to the frame 109 and/or theother ones of the heat pipe layers 106. This can allow the heat pipelayers 106 to expand/contract independently due to heat and/orirradiation—thereby reducing mechanical stress on the heat pipe layers106, the frame 109, and/or other components of the system 100.Similarly, all or a subset of the heat pipes in each of the heat pipelayers 106 can expand/contract independently due to heat and/orirradiation. For example, each of the wedge-shaped branches of the heatpipes 220 (e.g., the branch shown in FIG. 2B) or the heat pipes 520, 552(e.g., the branch shown in FIG. 5B) can move independently of the otherwedge-shaped branches in the heat pipe layer 106.

In some embodiments, to refuel the system 100, one or more of the heatpipe layers 106—and the fuel 226 attached to or otherwise associatedtherewith—can be removed and replaced and/or shifted to another locationwithin the stack of heat pipe layers 106. Similarly, in some embodimentsone or more of the independent branches of the heat pipes can beshifted/moved during a refueling process. In another aspect of thepresent technology, the modular nature of the heat pipe layers 106 canenable the heat pipe layers 106 to be manufactured/fabricated inparallel and subsequently assembled.

The following examples are illustrative of several embodiments of thepresent technology:

-   -   1. A nuclear reactor, comprising:    -   a heat pipe network including an evaporator region, an adiabatic        region, and a condenser region, wherein the heat pipe network        defines a plurality of flow paths having an increasing        cross-sectional flow area in a direction from the evaporator        region toward the condenser region; and    -   nuclear fuel thermally coupled to at least a portion of the        evaporator region, wherein the heat pipe network is positioned        to transfer heat received from the fuel at the evaporator region        to the condenser region.    -   2. The nuclear reactor of example 1 wherein the nuclear fuel is        directly attached to the evaporator region.    -   3. The nuclear reactor of example 2 wherein the nuclear fuel is        a uranium molybdenum alloy pressed onto the evaporator region.    -   4. The nuclear reactor of any one of examples 1-3 wherein the        nuclear reactor further comprises a liquid metal moderator at        least partially surrounding the evaporator region, and wherein        the liquid metal moderator is positioned to control a reactivity        of the nuclear fuel.    -   5. The nuclear reactor of any one of examples 1-4 wherein the        nuclear reactor further comprises a reflector positioned to        reflect neutrons resulting from fission of the nuclear fuel, and        wherein at least a portion of the heat pipe network extends        through the reflector.    -   6. The nuclear reactor of example 5 wherein the adiabatic region        extends away from the evaporator region through the reflector,        and wherein the condenser region is positioned outside the        reflector.    -   7. The nuclear reactor of any one of examples 1-6, further        comprising a heat exchanger thermally coupled to the condenser        region.    -   8. The nuclear reactor of any one of examples 1-7 wherein        individual ones of the flow paths branch into two or more        portions in the direction from the evaporator region toward the        condenser region.    -   9. The nuclear reactor of any one of examples 1-8 wherein        individual ones of the flow paths include (a) a first portion        proximate a central axis of the heat pipe network, (b) a pair of        second portions branching from the first portion, (c) a pair of        third portions branching from each of the second portions,        and (d) a pair of fourth portions branching from each of the        third portions.    -   10. The nuclear reactor of example 9 wherein the nuclear fuel is        thermally coupled to the first portion, the second portions, and        the third portions of the flow paths.    -   11. The nuclear reactor of example 10 wherein the nuclear        reactor further comprises a reflector positioned to reflect        neutrons resulting from fission of the nuclear fuel, and wherein        the fourth portions extend through the reflector.    -   12. The nuclear reactor of any one of examples 1-11 wherein the        heat pipe network includes (a) arterial flow paths extending        radially away from a central axis of the heat pipe network        and (b) off-branching flow paths extending circumferentially        about the central axis and connecting circumferentially adjacent        pairs of the arterial flow paths.    -   13. The nuclear reactor of example 12 wherein individual ones of        the off-branching flow paths include one or more evaporator        heads, and wherein the nuclear fuel is directly attached to the        evaporator heads.    -   14. A nuclear reactor, comprising:    -   a plurality of heat pipe layers, wherein individual ones of the        heat pipe layers include (a) a heat pipe network extending        radially outward from a central axis and (b) fissile material        thermally coupled to at least a portion of the heat pipe        network, and wherein the heat pipe network has an increasing        cross-sectional flow area in a radially-outward direction from        the central axis;    -   a reflector positioned to reflect neutrons resulting from        fission of the fissile material in a radially-inward direction        toward the central axis, wherein the heat pipe networks in        individual ones of the heat pipe layers extend at least        partially through the reflector; and    -   a heat exchanger thermally coupled to the heat pipe layers,        wherein the heat pipe layers are positioned to transfer heat        received from the fissile material to the heat exchanger.    -   15. The nuclear reactor of example 14 wherein the heat pipe        layers are stacked one over another.    -   16. The nuclear reactor of example 14 or example 15 wherein the        heat pipe layers are coupled to and supported by a frame.    -   17. The nuclear reactor of any one of examples 14-16 wherein the        reflector comprises a solid material, and wherein the heat        exchanger is positioned radially outside of the reflector.    -   18. The nuclear reactor of any one of examples 14-17 wherein the        reflector comprises a fluid, wherein the heat exchanger is        positioned at least partially within the fluid, and wherein the        fluid is in thermal contact with the fluid and the heat pipe        layers to transfer heat from the heat pipe networks to the heat        exchanger.    -   19. A method of forming a nuclear reactor component, the method        comprising: positioning a heat pipe in a pressure vessel;        providing a powdered nuclear fuel material on at least a portion        of the heat pipe; and increasing a pressure and a temperature in        the pressure vessel to solidify the nuclear fuel material and        directly attach the nuclear fuel material to the heat pipe.    -   20. The method of example 19 wherein the nuclear fuel material        is a fissile material.    -   21. The method of example 19 or example 20 wherein the nuclear        fuel material is uranium molybdenum alloy.    -   22. The method of any one of examples 19-21 wherein increasing        the pressure in the pressure vessel includes pumping an inert        gas into the pressure vessel.    -   23. The method of any one of examples 19-22 wherein providing        the powdered nuclear fuel material on at least the portion of        the heat pipe includes providing the powdered nuclear fuel        material on an evaporator region of the heat pipe, wherein the        heat pipe further includes a condenser region, and wherein the        heat pipe defines a flow path having an increasing        cross-sectional flow area in a direction from the evaporator        region toward the condenser region.

The above detailed description of embodiments of the present technologyare not intended to be exhaustive or to limit the technology to theprecise forms disclosed above. Although specific embodiments of, andexamples for, the technology are described above for illustrativepurposes, various equivalent modifications are possible within the scopeof the technology as those skilled in the relevant art will recognize.For example, although steps are presented in a given order, otherembodiments may perform steps in a different order. The variousembodiments described herein may also be combined to provide furtherembodiments.

From the foregoing, it will be appreciated that specific embodiments ofthe technology have been described herein for purposes of illustration,but well-known structures and functions have not been shown or describedin detail to avoid unnecessarily obscuring the description of theembodiments of the technology. Where the context permits, singular orplural terms may also include the plural or singular term, respectively.

As used herein, the phrase “and/or” as in “A and/or B” refers to Aalone, B alone, and A and B. To the extent any materials incorporatedherein by reference conflict with the present disclosure, the presentdisclosure controls. Additionally, the term “comprising” is usedthroughout to mean including at least the recited feature(s) such thatany greater number of the same feature and/or additional types of otherfeatures are not precluded. It will also be appreciated that specificembodiments have been described herein for purposes of illustration, butthat various modifications may be made without deviating from thetechnology. Further, while advantages associated with some embodimentsof the technology have been described in the context of thoseembodiments, other embodiments may also exhibit such advantages, and notall embodiments need necessarily exhibit such advantages to fall withinthe scope of the technology. Accordingly, the disclosure and associatedtechnology can encompass other embodiments not expressly shown ordescribed herein.

We claim:
 1. A nuclear reactor, comprising: a heat pipe networkextending radially outward from a central axis and including anevaporator region, an adiabatic region, and a condenser region, whereinthe adiabatic region is farther from the central axis than theevaporator region, wherein the condenser region is farther from thecentral axis than the adiabatic region, wherein the heat pipe networkdefines a plurality of flow paths having an increasing cross-sectionalflow area in a radial direction away from the central axis such that thecross-sectional flow area is greater along an entire length of thecondenser region than along an entire length of the evaporator region,and wherein individual ones of the flow paths branch into two or moreportions in the radial direction; and nuclear fuel thermally coupled toat least a portion of the evaporator region, wherein the heat pipenetwork is positioned to transfer heat received from the fuel at theevaporator region to the condenser region.
 2. The nuclear reactor ofclaim 1 wherein the nuclear fuel is directly attached to the evaporatorregion.
 3. The nuclear reactor of claim 2 wherein the nuclear fuel is auranium molybdenum alloy pressed onto the evaporator region.
 4. Thenuclear reactor of claim 1 wherein the nuclear reactor further comprisesa liquid metal moderator at least partially surrounding the evaporatorregion, and wherein the liquid metal moderator is positioned to controla reactivity of the nuclear fuel.
 5. The nuclear reactor of claim 1wherein the nuclear reactor further comprises a reflector positioned toreflect neutrons resulting from fission of the nuclear fuel, and whereinat least a portion of the heat pipe network extends through thereflector.
 6. The nuclear reactor of claim 5 wherein the adiabaticregion extends away from the evaporator region through the reflector,and wherein the condenser region is positioned outside the reflector. 7.The nuclear reactor of claim 1, further comprising a heat exchangerthermally coupled to the condenser region.
 8. A nuclear reactor,comprising: a heat pipe network including an evaporator region, anadiabatic region, and a condenser region, wherein the heat pipe networkdefines a plurality of flow paths having an increasing cross-sectionalflow area in a direction from the evaporator region toward the condenserregion, and wherein individual ones of the flow paths include (a) afirst portion proximate a central axis of the heat pipe network, (b) apair of second portions branching from the first portion, (c) a pair ofthird portions branching from each of the second portions, and (d) apair of fourth portions branching from each of the third portions; andnuclear fuel thermally coupled to at least a portion of the evaporatorregion, wherein the heat pipe network is positioned to transfer heatreceived from the fuel at the evaporator region to the condenser region.9. The nuclear reactor of claim 8 wherein the nuclear fuel is thermallycoupled to the first portion, the second portions, and the thirdportions of the flow paths.
 10. The nuclear reactor of claim 9 whereinthe nuclear reactor further comprises a reflector positioned to reflectneutrons resulting from fission of the nuclear fuel, and wherein thefourth portions extend through the reflector.
 11. The nuclear reactor ofclaim 1 wherein the heat pipe network includes (a) arterial flow pathsextending radially away from a central axis of the heat pipe network and(b) off-branching flow paths extending circumferentially about thecentral axis and connecting circumferentially adjacent pairs of thearterial flow paths.
 12. The nuclear reactor of claim 11 whereinindividual ones of the off-branching flow paths include one or moreevaporator heads, and wherein the nuclear fuel is directly attached tothe evaporator heads.
 13. A nuclear reactor, comprising: a plurality ofheat pipe layers, wherein individual ones of the heat pipe layersinclude (a) a heat pipe network extending radially outward from acentral axis and (b) fissile material thermally coupled to at least aportion of the heat pipe network, and wherein the heat pipe network hasan increasing cross-sectional flow area in a radially-outward directionfrom the central axis; a reflector positioned to reflect neutronsresulting from fission of the fissile material in a radially-inwarddirection toward the central axis, wherein the heat pipe networks inindividual ones of the heat pipe layers extend at least partiallythrough the reflector; and a heat exchanger thermally coupled to theheat pipe layers, wherein the heat pipe layers are positioned totransfer heat received from the fissile material to the heat exchanger.14. The nuclear reactor of claim 13 wherein the heat pipe layers arestacked one over another.
 15. The nuclear reactor of claim 13 whereinthe heat pipe layers are coupled to and supported by a frame.
 16. Thenuclear reactor of claim 13 wherein the reflector comprises a solidmaterial, and wherein the heat exchanger is positioned radially outsideof the reflector.
 17. The nuclear reactor of claim 13 wherein thereflector comprises a fluid, wherein the heat exchanger is positioned atleast partially within the fluid, and wherein the fluid is in thermalcontact with the fluid and the heat pipe layers to transfer heat fromthe heat pipe networks to the heat exchanger.